1. Field of the Invention
The invention relates generally to devices which read information from a magnetic medium. More particularly, the invention concerns a user programmable control circuit and user interface to optimize write-to-read settling times of such a device.
2. Description of the Prior Art
Magnetic recording devices are used to write information to a magnetic medium either in analog or digital form. The magnetic medium is typically a tape, such as used in the well known compact cassette or known high density drives for computers which are used for backup purposes. Another common form is a disc, such as a floppy disc or a hard disc for storing programs and data in a computer.
The read channel for a magnetic recording device includes a sensor in the form of a magneto-resistive head in close proximity with the magnetic medium. When the magnetic material is moved relative to the sensor, a flux is induced in the sensor in dependence of the local orientation of the magnetic material, thereby generating an information signal which can be amplified and then decoded.
When information is written in digital form, such as for computer data storage or digital recording of music, a current is generated by a write channel and passed through a thin film head in one direction to write a binary xe2x80x9c0xe2x80x9d and in the opposite direction to write a xe2x80x9c1xe2x80x9d. When the medium is read by the sensor, or read head, the portions recorded with a binary xe2x80x9c0xe2x80x9d will induce a current in the head in the one direction and portions recorded with a binary xe2x80x9c1xe2x80x9d will induce a current in the opposite direction, which is then decoded by a bit detector.
Hard disk drives typically include multiple magnetic discs, or platters, each side of which are used for reading and writing information. The read heads and write heads are mounted on arms positioned at each side of the disk, and in modern systems, servo patterns are embedded in radial patterns on the disk. Reading and writing are interchanged in conformance with the servo patterns.
Write-to-read settling time is an important specification because it impacts directly with drive capacity. Write-to-read settling time is defined to be the time required for the read channel to settle and be able to read the next servo field after the write channel has stopped writing. The gap on the magnetic disk between the end of the write sector and the start of the servo field is wasted area and hence should be minimized.
The disturbance to the read channel during write-to-read transition comes mainly from three sources. The first source is a flexible cable which is conventionally used to electrically couple the integrated circuit which embodies the read and write channel circuitry to the write heads and read sensors. This cable has multiple conductors in close proximity, which is a source of interference. The circuitry for the write channel and the read channel are typically embodied on a single integrated circuit, referred to in the art as a xe2x80x9cpreamplifier ICxe2x80x9d. The preamplifier IC will have several read and write channels, grouped in read/write pairs for each read/write head combination in the device. The close physical proximity of the read channel circuitry and the write channel circuitry on the integrated circuit is a source of undesired capacitive coupling between the write and read channels. A third source of interference is from the connection pens, bond wires and bond pads on the integrated circuit, which may also be a source of unwanted coupling. The disturbances from each of these sources needs to be suppressed from propagating to the output of the read channel, so as to avoid errors in decoding the information read from the disc. On the other hand, excess suppression will also interfere with the proper reading of actual data.
The exact duration and nature of write-to-read disturbances are somewhat unpredictable. It depends on the assembly of the heads, sensors and flexible cable, as well as other sources as described above, and hence varies between models of hard disk drives assembled by a manufacturer of such devices. When designing a preamplifier IC, the designer must model the interference sources, based on design data provided by the disk drive manufacturer as well as past experience (if any) with other models of that manufacturer, to design a suitable filter for interference suppression for the read channel. After fabricating the preamplifier IC, the IC is tested in the intended hard disk of the manufacturer. If there is too little suppression or too much suppression, unacceptable read errors will occur and the designer will have to redesign the filters in the IC, necessitating changes to the mask set for the IC as well as re-fabrication of the preamplifier IC with the revised filters. Such an iterative process undesirably lengthens the design cycle, increasing costs for both the IC manufacturer and the manufacturer of the hard disk drive or other magnetic storage device.
Even after arriving at a suitable design for the preamplifier IC, the hard disk drive manufacturer still faces problems with chip-to-chip variations in the pre-amplifier ICs, as well as drive-to-drive variations in the assembly of the read sensors, write heads, flex cables and their interconnection with the preamplifier IC. While all elements may be xe2x80x9cwithin specxe2x80x9d, the combination of several elements near their outer tolerance limits will often yield a hard disk drive with a read error rate outside of acceptable limits.
In the present state of the art, suitable tools are not available to the IC manufacturer to avoid the above-mentioned iterations in the design cycle nor for the drive manufacturer to easily fix read error problems arising from normal chip-to-chip variations and variations in the head/sensor/cable interconnect.
Accordingly, there is a need for a magnetic storage apparatus in which the manufacturer or other user may optimize the performance of the read channel before, during or after assembly. There is also a need for a magnetic storage apparatus with a control device in the read channel which is user programmable to reduce write-to-read settling time in an efficient manner. Additionally, there is a need for an integrated circuit having a read channel which can be programmed by the manufacturer of the integrated circuit or the manufacturer""s customer to optimize write-to-read settling time.
These and other objects of the invention are satisfied according to a first aspect of the invention in which a magnetic information storage apparatus includes a write channel for writing information to a magnetic medium and a read channel for reading information from the magnetic medium. The read channel includes a sensor which generates an information signal in response to information stored on the magnetic medium and a signal path which amplifies the information signal from the sensor and provides an amplified information signal. The signal path has at least one control device which controls a zero of the signal path in a time-dependent manner, the control device being programmable to control the time dependent response of the zero.
When transitioning from a write mode in which the write channel is active to a read mode in which the read channel is active, the transition typically induces a write-to-read disturbance in the read channel. By placing an externally programmable control device in the read channel, the read channel can be adapted to variations which occur from model-to-model of a magnetic storage apparatus of a particular manufacturer. Alternatively, the read channel can be adapted to differences between magnetic storage devices of different manufacturers, so that a supplier of preamplifier IC""s can satisfy the needs of multiple customers with only one integrated circuit design. Finally, it is now feasible for a manufacturer of magnetic storage devices to optimize performance to account for unit-to-unit variations within a model type.
Favorably, the programmability is accomplished via a user interface, such as a serial interface, which allows a user to enter values for one or more variables which control the control device.
According to a second aspect of the invention, the control device includes a filter having the zero, the filter varying the zero from a first frequency to a second, lower frequency in a time-dependent manner related to a time duration of the write-to-read disturbance. The inventors have found that in a magnetic storage device for high speed applications the write-to-read disturbance is characterized by an initial high peak and a relatively long tail in which the disturbance decays from the peak to a minimal value. By varying the frequency of the zero (i.e. the low corner frequency) in the aforementioned manner, the attenuation in the filter is varied in a manner related to the amplitude of the disturbance so that high filtering occurs when the amplitude of the disturbance is high and low filtering occurs when the amplitude of the signal is low. This optimizes the reduction of the settling time, and avoids read errors which would occur if the filtering was, for example, fixed at a high level.
According to another aspect of the invention, the zero in the read channel is provided by a low pass filter in a feedback path of the read channel. The low pass filter has a pole which is variable, for example by a variable capacitance, to vary the location of the zero in the forward path. In a read channel having a plurality of gain stages, it is desirable that the time-dependent zero is in the xe2x80x9cback endxe2x80x9d of the signal path, i.e. closer to the output of the signal path than the first gain stage. This helps in reducing DC offset of the read channel. Offset is the difference between the differential DC component at the output when the differential DC component at the input is zero, and is caused by mismatches of components of the amplifier stages, branch currents, and parasitic resistances in wiring.
According to yet another aspect of the invention, the user interface enables programming of at least one of: (a) the duration of time the zero is at the first frequency, (b) the rate of change of the zero from the first frequency to the second frequency, and (c) a gain of the control device.
According to still another aspect of the invention, the signal path includes a first gain stage coupled to the sensor which forms the control device. A zero of the read channel is controlled by a transconductance stage with a variable pole in a feedback path of the first gain stage, which in turn is varied by varying a dc gain of a feedback amplifier in the feedback path. In an embodiment, a current bias circuit coupled to the transconductance stage varies the gain thereof, and a controllable pulse generation circuit coupled to the current bias circuit generates a pulse to control the bias current supplied by the bias circuit to the transconductance stage.
According to another aspect of the invention, a characteristic of the pulse is programmable to control at least one of: (a) a duration of the low corner frequency at a high initial frequency, (b) an amplitude of the bias current to the feedback transconductance stage, which controls gain and hence the pole of the feedback amplifier, and (c) the transition of the zero from the high frequency to the low frequency.
Favorably, optimum control of the settling time of the read channel is provided in a magnetic storage apparatus having a control element in the first gain stage as well as in a xe2x80x9cback-endxe2x80x9d stage as described above.
Another aspect of the invention is an integrated circuit for use in a magnetic storage device includes a read channel with one or more of the above mentioned features.
These and other objects, aspects, features and advantages of the invention will become apparent with reference to the following detailed description and the drawings.